1. Field of the Invention
This invention relates to ophthalmic lenses in general and is more particularly concerned with improvements in progressive power lenses for the correction of presbyopia.
2. Description of the Prior Art
The use of progressive power lenses for the correction of presbyopia has become increasingly popular in recent years. In addition to their obvious cosmetic appeal, progressive lenses provide significant functional benefits to the patient, namely a continuous range of focal powers and an unobstructed visual field. Such advantages are, however, partially offset by peripheral astigmatism and distortion aberrations that are unavoidably present in all progressive lenses. The design of progressive lenses thus naturally centers on reducing the unwanted aberrations to minimum effect.
It is generally recognized that the aberrations can be minimized permitting them to extend over broad areas of the lens including, for example, the peripheral portions of the near vision level. This, of course, implies a sacrifice of acuity in those peripheral areas. However, virtually all modern commercial progressive lenses make use of the principle of extended-area aberration control. U.S. Pat. Nos. 3,687,528 and 4,056,311 are exemplary.
It is not enough merely to state that the aberrations shall occupy extended areas of the lens. The manner of their distribution within those areas is critically important. Badly distributed aberrations can undo the potential advantage gained by sacrificing acuity within the peripheral areas. For example, if a high value is placed on the requirement of orthoscopy (i.e. the maintence of horizontals and verticals in visual field), the designer shapes the peripheral aberrated zones in such a way that the component of vertical prism along horizontal lines remains constant. The corrected peripheral areas, however, must be joined to the central portion of the intermediate area, and the latter cannot be corrected to preserve orthoscopy. Therefore, a blend zone must be interposed between the inner and outer areas. The blend must not be made too abruptly or the visually annoying condensation of aberration within the blend zone will overpower and may effectively negate the advantage of orthoscopy gained at the lens periphery.
Progressive lenses heretofore designed for preservation of orthoscopy do not directly address the requirement of uniform distribution of aberrations and it is a principal object of this invention to fully exploit a technique of extended-area aberration control to achieve smooth and natural optical effect.
More particularly, there is the objective of providing a progressive power ophthalmic lens with progressive surface designed to insure a uniform distribution of aberrations and a smooth optical effect with orthoscopy at least approximately preserved in lateral peripheral areas of the lens and without accrual of strong aberrations elsewhere in the lens.
Still another object is to provide a natural flow of optical lens power which will be readily accepted by emerging and advanced presbyopes alike.